1. Field of the Invention
The invention relates to an ultrasonic probe, with sectorial scanning, designed for transmitting ultrasonic bursts in different directions and receiving corresponding echoes; it relates more particularly to the means for angular location of the mobile assembly of the probe. The invention also relates to an improved echography installation comprising such a probe.
2. Description of the Prior Art
A conventional ultrasonic probe comprises a mobile assembly mounted rotatably about a shaft in a case and adapted to radiate an ultrasonic beam outwardly of the case and to receive the corresponding echoes so as to provide the complete exploration of a given sector of a sample, for reconstituting an image thereof. Thus, to each transmission-reception sequence made for a given angular position of the mobile assembly, there corresponds a "line" of the subsequently reconstituted image. The mobile assembly may be formed by the piezo-electric transducer itself or by a mirror reflecting the ultrasonic beam emitted by a fixed transducer. Sectorial scanning may be obtained by causing the mobile assembly to oscillate about its axis; an equivalent result may also be obtained by means of a cylindrical mobile assembly having several transducers and being rotated at a constant speed.
One of the problems arising with these types of probes (principally the oscillating probe) is the precise location of the angular position of the mobile assembly at any time, so as to always trigger off the transmission bursts or "shots" at the same angular positions, is difficult to determine. The accuracy of the mobile assembly during the respective positions of the different shots required for reconsituting an image forms an important characteristic affecting the quality of the image. Good accuracy allows the different lines to be correctly situated with respect to each other in the image and also the positions of the lines to be stabilized from one image to another. Attempts have been made to resolve this problem by linearizing as much as possible the oscillating movement of the mobile assembly by means of a servo-control chain looped to the drive motor of the mobile assembly. Another attempt consists simply in angularly locating the mobile assembly and triggering off the bursts from signals representative of this location. In both cases, the devices which have been associated with the motor (precision potentiometers, for example) either for providing the servo-control error signal or for giving concrete form to the angular location, are cumbersome and subject to drifting in time. The space occupied by such a device requires that it be disposed at a certain distance from the mobile assembly. The coupling is then necessarily provided by means of a mechanical change of direction system. Whatever the quality of execution of this system, the free motions can never be completely eliminated and play a particularly critical role in probes with an oscillating mobile assembly, since free motions come into play at each reversal of the direction of movement. This is one of the main causes of wear and of derangement of this type of probe.
Moreover, the accuracy of all such location devices supplying co-called "analog" signals is intrinsically limited so that use of incremental coders providing information usable by digital processing systems, appears more and more desirable. The invention provides a solution for the above-mentioned problems by proposing the adaptation of an incremental coder to a sectorial scanning mechanical probe.